Plasmids

Plasmids

Although they are rare in eukaryotic cells (other than some yeasts), plasmids are quite common inside bacteria. Usually unnecessary for host survival, these DNA molecules nonetheless can enable their host bacteria to kill other bacteria, resist the effects of antibiotics, or serve as tiny industrial workhorses in waste management. There are many different kinds of plasmids, which can be categorized according to their function.

Conjugative plasmids can be transmitted by conjugation from one bacterium to another, such as the E. Coli F plasmid described in the preceding section. Non-conjugative plasmids are not transmitted by conjugation, but can be transferred by transduction or, with special procedures, by transformation. Non-conjugative plasmids can also be transmitted to a recipient cell by the co-operative action of conjugative plasmids that may coexist with them in the same cell.

Bacteriocinogenic plasmids contain a gene that directs the host bacterium to synthesize a bacteriocin. A bacteriocin is a protein that kills bacteria belonging to the same or closely related species and lacking the plasmid. Interestingly, a bacterium that makes a particular bacteriocin is not killed by other bacteriocins. There are many different bacteriocins, including those produced by bacteria normally found in the intestine. They are used in medical bacteriology to help identify the different subgroups of bacteria such as e. Coli and Pseudomonas species. Such tests help determine whether several outbreaks of an infectious disease are due to one or more particular strains of bacteria.

Other plasmids carry genes for resistance to antibiotics. Each antibiotic resistance gene in these R plasmids codes for an enzyme that destroys or inactivates a particular antibiotic. For example, most strains of staphylococcus aureus isolated in hospitals today contain an R plasmid with a gene that codes for -lactamase, an enzyme which inactivates many penicillins. Some R plasmids are conjugative plasmids, readily transferring antibiotic resistance genes to other bacterial cells by conjugation.

The ability of a conjugative R plasmid to confer upon recipient bacteria a simultaneous resistance to several antibiotics can frustrate medical treatment. In many instances, the plasmid-mediated resistance of various bacteria to frequently used antibiotics such as ampicillin, chloramphenicol, tetracycline, kanamycin and streptomycin makes it difficult to treat infections successfully. Indiscriminate use of antibiotics to treat patients can increase the incidence of antibiotics-resistant bacteria.

Plasmids that code for important degradative enzymes are called dissimilation plasmids. This plasmid type is responsible for the ability of certain Pseudomonas species to break down difficult to degrade industrial solvents such as toluene and xylene. A combination of several plasmids, when transferred to Pseudomonas bacteria, allows the bacteria to break down complex hydrocarbons and other compounds present in crude oil.

These bacteria have potential use for treatment of environments contaminated by oil spills.

Some plasmids can replicate only in a few species of closely related bacteria, while others have a much broader host range. An example of the latter is conjugative plasmids called incP plasmids, which can be transmitted to and replicated in nearly all species of gram-negative bacteria. There also are some plasmids that are very selective about their neighbours; they cannot coexist in the same cell with certain other plasmids. On the basis of this, plasmids have been classified into various incompatibility groups, where those in one group cannot coexist with those in other groups.

Treating bacterial cells with certain chemical agents, such as certain dyes, or with high temperatures can sometimes free them of plasmids. Cells thus treated are said to be “cured”. For example. E.coli can be “cured” by using the dye acridine orange.

Based on function plasmids are of following types:

R plasmid:

This plasmid contains genes that provide resistance to the antibiotics. Some plasmids code enzyme that hydrolyzes the antibiotic whereas some plasmids code enzyme that chemically modified antibiotic so that antibiotically becomes inactive.

Virulence plasmid:

This plasmid contains the gene that codes for the virulence factor. Therefore, these plasmid increases the pathogenicity of microorganisms. For.eg exotoxin production many bacteria is mediated by this plasmid.

F-Factor or fertility factor:

This plasmid helps in the conjugation of bacteria. It synthesizes sex pilus between two bacteria during conjugation.

Metabolic plasmid:

The plasmid helps in various metabolic activities. For eg. N₂ fixation and root nodulation genes of rhizobium are present in this plasmid.

Bacteriocinogenic factors:

This plasmid contains the gene for the production of bacteriocin. Bacteriocin is a protein produced by bacteria that kills the same or other closely related bacteria.

Based on the property, plasmids are of following types.

1. Conjugative plasmid: This plasmid is transferred from donor bacteria to the recipient bacteria during conjugation. It contains transferase gene (tra).
2. Non-conjugative plasmid: these plasmids lack ‘tra’ gene and hence are not transferred during conjugation.
3. Episome: Plasmid which can live either freely or gets integrated into the host chromosome is called episome.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.